Because the transmitter section of the transceiver typically outputs signals at a power level much higher than that of signals received by the receiver section of the transceiver, the receiver section may suffer from self-interference from the transmitter section in many situations. This self-interference, sometimes called crosstalk, self-jamming, or transmitter leakage, results from coupling of the transmitted signal to the receiver section principally through reflections caused by antenna or impedance mismatch.
The block diagram for a representative transceiver is illustrated in FIG. 1 based on digital waveform generation and digital IF signal processing. The transmit signal in this case is created using direct digital synthesis (DDS) which is amplified to 5 W in a linear power amplifier and coupled to the whip antenna through a transmit/receiver coupler and narrow band antenna match. A signal received by the antenna is conveyed by the coupler to a double conversion receiver where it is processed to a digital IF/baseband signal. The received signal is up-converted to a 45 MHz IF using a high dynamic range balanced mixer that is driven by a 47-75 MHz synthesizer. A two-pole crystal filter selects the desired HF signal component. The Analog to Digital Converter (“ADC”) is driven by a narrow-band low-noise amplifier.
There are several well known methods for implementing the transmit/receive coupler. A switch is employed in a wide range of applications where talk and listen do not occur simultaneously, as with most HF transceivers. A second well know method of sharing an antenna is to use a frequency selective diplexer. Such circuits generally work well for frequency separations of 10 MHz or more. Diplexers can be built using resonant cavities that work at VHF frequencies (>100 MHz) but do not meet the SWAP requirements for a man-portable transceiver (size≈1 cubic foot, weight≈5 lbs).